Add an arm64 sha512 feature check

[andrew]

Summary

This is only available in recent toolchains. Check for it so we can
use it in libmd.

[emaste]

share/mk/bsd.compiler.mk

258

If this is the case retpoline should be here too, but instead I think we could just drop this comment, list aarch64-sha512 in the list at line 22, and add a MD annotation there.

[andrew]

Update based on feedback